1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to determining a deviation of an optical test surface from a target shape using an interferometer. Apparatuses and methods consistent with the present invention also relate to measuring a local distribution of an optical property over an optically effective area of an optical test object using an interferometer. In addition, apparatuses and methods consistent with the present invention relate to producing an optical element, the optical element itself, imaging optics comprising the optical element, and a projection exposure tool.
2. Description of the Related Art
An example of an interferometric apparatus of the type used is described e.g. in WO 2006/077145 A2. This apparatus comprises an interferometer for producing an incoming beam, the wave front of which is hereupon adapted to the desired shape of the optical surface. The wave front of the adapted incoming beam is analyzed interferometrically after reflection at the optical surface, and the deviation of the actual shape of the optical surface from the target shape of the latter is thus determined.
The optical element comprising the optical test surface is, for example, an optical component such as a lens or a mirror. These types of optical components are used in optical systems, such as telescopes used in astronomy, or in imaging systems as used in lithographic processes. The success of this type of optical system is substantially determined by the precision with which the optical components of the latter can be produced and then be processed, such that the surface shapes of the latter respectively correspond to a target shape which was specified by a designer of the optical system when designing the latter. Within the framework of this type of production it is necessary to compare the shape of the processed optical surfaces with the target shape of the latter, and to determine any differences and deviations between the finished surface and the desired surface. The optical surface can then be processed in those regions where differences between the processed surface and the desired surface exceed, for example, pre-specified threshold values.
Generally, interferometers are used for very high precision measurements on optical surfaces. A related art interferometer arrangement for measuring an optical surface typically comprises a coherent light source and interferometer optics in order to produce an incoming light beam which strikes the surface to be measured such that wave fronts of the incoming beam have at locations of the surface to be measured respectively a same shape as the target shape of the surface to be measured. In this type of situation the light of the incoming beam strikes each location of the surface being measured essentially orthogonally and is then reflected back from the latter in itself. The reflected back light is then overlaid with reference light which has been reflected by a reference surface. Deviations between the shape of the surface measured and its target shape can then be established from interference thus produced.
Whereas spherical wave fronts can be produced for measuring spherical optical surfaces with a relatively high degree of precision by using related art interferometer optics, advanced techniques are required in order to produce incoming beams the wave fronts of which are aspherical so that the light at each location of an aspherical optical surface to be measured strikes the latter orthogonally. In order to produce these types of incoming light, beam optics are used which are called zero lenses, K systems or compensators. Background information with regard to these zero lenses or compensators can be found in Chapter 12 of the text book by Daniel Malacara, Optical Shop Testing, 2nd edition, Wiley interscience Publication (1992).
The precision of deviation measurements achieved by current systems is often not satisfactory considering the tight tolerances required for high quality optics, in particular in a case in which the target shape of the test surface deviates from rotational symmetry.